1. Field of the Invention
The present invention relates to a metal surface inspection device. More particularly, the present invention relates to a metal surface inspection device suitable for use in inspecting the surface of metal rings which are parts that constitute a V-belt type of a Continuously Variable Transmission belt (hereinafter referred to as “CVT belt”).
2. Description of the Related Art
Conventionally, there is a known CVT belt structure which laminates a plurality of thin metal rings in a stack of approximately 0.2 mm in thickness to which steel elements are consecutively attached.
FIG. 5 is an outline view of a CVT belt. In this diagram, a CVT belt 1 is constructed by assembling two laminated bands of a layered belt 2 that contain a stack of a number of metal rings 2a (for example, a laminated band composed of about 12 endless layers) which are supported by a layered element 3 composed of a large number of steel elements 3a (for example, about 400 elements).
In this manner, the structure of the CVT belt 1 is produced through the following processes:
(1) First, a ring-shaped drum is formed by welding together the ends of a thin sheet of ultrahigh strength steel, such as maraging steel.
(2) Next, the drum is cut into round slices of a predetermined width and rolled to create metal rings 2a of a basic peripheral length.
(3) Next, after performing a solution treatment, etc. to the above-mentioned metal rings 2a, the necessary peripheral length (namely, the peripheral difference between the inner and outer periphery) corresponding to the stacked layers of the CVT belt 1 using a “peripheral length correction device” is accomplished. Furthermore, aging treatment, nitride treatment, etc. are performed to increase the hardness of the metal rings 2a. 
(4) Lastly, the metal rings 2a are laminated after undergoing the above-mentioned process (3), the steel elements 3a are consecutively attached and the CVT belt 1 is completed.
Naturally, since these metal rings 2a undergo the above-mentioned processes (such as manufacture of the drum, cutting, rolling, solution treatment, peripheral length correction, aging treatment, nitride treatment, etc.), partial defects occur, such as abrasions and indentations on the front and rear end faces of the metal rings 2a. 
As an inspection method for such defects, there is a process for factory workers to determine the existence of abrasions, indentations, etc. by visually observing the front and rear end faces of the metal rings 2a preceding the CVT belt manufacturing process (4), namely by directly viewing parts or using a magnifying glass. However, in this antiquated method there is the drawback of being inefficient due to the fact that human error rate is always higher than an automated process. Thus, satisfactory reproducibility and reliability are not routinely acquired.
As for conventional prior art which is applicable to the surface inspection of the above-mentioned metal rings 2a, for example, Japanese Laid-Open Patent Application No. H11-248637 (1999) titled “DEFECT DETECTING DEVICE” (hereinafter denoted as “conventional prior art device”) is known.
Particularly noteworthy, the principle configuration of this conventional prior art device comprises an inspection light source which illuminates the inspectable surface and at least two light guiding paths (optical fiber) which guide reflected light from the inspectable surface to the light reception segments, and the spacing arrangement of the two light guiding paths is separated at a slight distance. Thus, in such a configuration when an inspectable surface does not have a defect, etc., the reflected light guided by the two light guiding paths is supplied to the light reception segments at substantially the same intensity. On the other hand, when an inspectable surface has a minor defect, since there is a decline (light intensity decline by diffused reflection) in the reflected light of an applicable defective part, a difference occurs in the light of the light guiding paths and the existence of a defect can be automatically discriminated from the amount of this difference.
However, although the above-mentioned conventional prior art is a beneficial device from the viewpoint of being able to automatically discriminate whether or not a defect exists on an inspectable surface, there is a drawback in which the inspection process excessively detects for example “mackerel skins” as manufacturing defects which do not impair the durability of the metal belt.
Here, “mackerel skins” is a common name referring to an abrasion with gloss which appears continuous or with a certain amount of length in the circumference direction of a metal belt. In nearly all cases, the entire abrasion is a smooth surface and the flaw does not impair the durability of a metal belt. Consequently, this type of flaw can be ignored as having no impact on the finished product. Thus, it is a waste of resources to exclude metal rings which have such a mackerel skin as a defective part and not preferred in terms of manufacturing cost.
Therefore, the object of the present invention is to provide an inspection device for metal rings which does not perform excessive detection of a mackerel skin as a detrimental flaw.